72) Amino acids are building blocks of proteins. These compounds contain at leas

Question

72) Amino acids are building blocks of proteins. These compounds contain at least one amino group (NH2) and one carboxyl group (COOH). Consider glycine (NH.CH.CoO. Depending on the p of the solution, glycine can exist in one of three possible forms Fully protonated: NH,-CH, COOH Dipolar ion: NH,-CH.-COO Fully ionizedt: NH CH.-Ccoo Predict the predominant form of glycine at pH 1.0, 7.0, and 12.0. The pk, of the carboxyl group is 2.3 and that of the ammonium group (-NH) is 96. 73) A 25.0-mL of 0.20 M HF solution is titrated with a 0.20 MNaOH solution. Calculate the volume of NaO solution added when the pH of the solution is (a) 2.85, (b) 3.15, (e) 11.89. Ignore salt hydrolysis. (Hint: The Ks value for HF is 7.1 x 10, hence, pKa-3.15) 74) One way to distinguish a buffer solution with an acid solution is by dilution. (a) Consider a buffer solution made of 0.500 M CHsCOOH and 0.500 MCH,COONa. Calculate its pH and the pH after it has been diluted 10- fold. (b) Compare the result in (a) with the pHs of a 0.500 MCHCoOH solution before and after it has been diluted 10-fold. 75) A 40.0 mL sample of 0.100 M HCl is titrated with 0.100 M NaOH. Calculate the pH after the addition of each of the following volumes of NaOH: (a) 0.0 mL, (b) 10.0 mL, (e) 20.0 mL, (d) 40.0 mL, (e) 60.0 mL. A plot of the pH of the solution as a function of the volume of added titrant is known as a pH titration curve Using the available data points, plot the pH titration curve for the above titration. 76) A 40.0 mL sample of 0.100 M acetic acid (CHsCOOH) is titrated with 0.100 M NaOH. Calculate the pH after the addition of each of the following volumes of NaOH: (a) 0.0 mL, (b) 20.0 mL, (c) 30.0 mL, (d) 40.0 mL, (e) 60.0 mL. A plot of the pH of a solution as a function of the volume of added titrant is known as a pH titration curve. Using the available data points, plot the pH titration curve for the above titration. H[CHs [CH,COOH -1.8 × 10-5) (Ka = 77) Calculate the pH of a buffer solution that is 0.20 M in HCN and 0.12 M in NaCN. Will the pH change if the solution is diluted by a factor of 2? Explain. K, for HCN is 4.9x 10-10 78) When the concentration of a strong acid is not substantially higher than 1.0 x 10 M, the ionization of water must be taken into account in the calculation of the solution's pH. (a) Derive an expression for th strong acid solution, including the contribution to [H.] from H2O. (b) Calculate the pH of a 1.0 x 10 M HCI solution. e pH of a When a typical diprotic acid H2A (Kai -1.2x104 K-2.1 x 10-) is titrated with NaOH, the principal A-containing spec can occur in an aqueous solution of the equilibrium constants for all the four proton-transfer reactions and show the principal reaction having the largest equilibrium constant among the four possible reactions.) Calculate the pH at the first equivalence point ies at the first equivalence point is HA. By considering all four proton-transfer reactions that show that the principal reaction i ) Problems in the Homework Assignments for Chapters 14, 15 and 16

Explanation / Answer

72) Given that the pKa of the carboxyl group is 2.3 and that of the amine group to be 9.6, it can be deduced that at pH = 2.3 exactly 50% of glycine will be fully protonated and 50% as zwitterion (dipolar ion). Similary at a pH of 9.6 there is a 50% population of fully ionized glycine.

Thus at a pH of 1 which is highly acidic, glycine exists in its fully protonated form; at pH = 7, the predominant species will be the dipolar form and at pH of 12, the predominant form will be the fully ionized form.

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